In a liquid metal cooled fast neutron nuclear reactor, the liquid metal constitutes the primary fluid permitting the cooling of the fuel element assemblies which give off heat and which form the core of the reactor.
The heat from the core is taken off by the liquid metal, generally liquid sodium, which is pumped to the bottom of the core by the primary pumps, which may be immersed in the liquid sodium filling the reactor vessel in the case of an integrated nuclear reactor, or be disposed outside the vessel in the case of a loop type nuclear reactor.
The liquid sodium heated by the reactor core enters heat exchangers permitting the heating of a second liquid metal constituting the secondary fluid, which in turn is used to produce steam in the steam generators.
On leaving the heat exchangers, the cooled primary fluid enters a zone of the vessel where, with the aid of the primary pumps, it is re-injected at the base of the reactor core.
The sodium cooled in the heat exchangers, i.e., cold sodium, is nevertheless at a temperature higher than 300.degree., so that the primary pumps undergo expansions when the nuclear reactor is in operation.
Furthermore, the temperature of the primary fluid is not constant in the course of the operation of the reactor, and consequently the expansion of the different parts of the pump is variable.
In addition, the pumps and their accessories undergo vibrations, which are in particular due to the circulation of the liquid sodium at high speed.
In nuclear reactors of the integrated type, and in particular, in which the primary pumps inject the liquid metal into ducts joined to the core support, it is necessary to assure continuity of circulation of this liquid sodium between the pump and the duct joined to the support, and at the same time to permit displacements of the pump relative to the core support or bed on which the fuel element assemblies rest and which enables them to be supplied with sodium.
In order to carry out these functions, use is generally made of a connecting sleeve having its ends fixed on the pump delivery duct and on the bed respectively, in a non-rigid manner, and leading into the duct joined to the bed.
A cylindrical sleeve of this kind nevertheless does not enable the speed of flow of the liquid metal to be controlled before it is injected into the duct joined to the bed.
Moreover, in order to permit transverse displacements relative to the flow, it is necessary to provide a universal joint arrangement for the entire pump, thereby complicating the construction and the fastening of the latter.
Finally, it is not possible to mount the pump sufficiently flexibly to absorb the displacements and at the same time sufficiently rigidly and with sufficient mechanical strength to withstand the various stresses (particularly earthquakes).